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March l5, 1960 c. F, QUA-rE Re# 24,794

LOW NOISE VELOCITY MODLATION TUBE Original Filed June l2, 1952 4SheetsSheet 1 o o .000000000000000 o oo.

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Lon NGISE VELOCITY MODULATION TUBE Original Filed June 12;` 1952 March15, 1960 lc. F. QUATE Re. 24,794

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2' Il; al 4l ''a' /lo 2o 3.040610'6'0700 7C /NVENTOR C. QUA TE ,ATTORNEYUnited States Patent() LOW NOISE VELOCITY MODULATION TUBE Calvin F.Quate, Berkeley Heights, NJ., assignor to Bell Telephone Laboratories,Incorporated, New York, N Y., a corporation of New York Original No.2,792,518, dated May 14, 1957, Serial No. 293,186, June 12, 1952.Application for reissue April 4, 1958, Serial No. 730,034

22 Claims. (Cl. S15-3.5)

Matter enclosed in heavy brackets appears in the original patent butforms no part of this reissue speeltication; matter printed in italicsindicates the additions made by reissue.

This invention relates to microwaves devices and more particularly tosuch devices which employ velocity modulation of an electron stream inaccordance with signal information to secure signal amplication.

A general object of the invention is to improve the noise figure of suchdevices.

A more specific object is to reduce the effect of the space charge waveswhich are set up by the thermal liuctuations at the source of electronstream and which propagate to the point of signal modulation of theelectron stream.

The invention has primary application to velocity modulation deviceswhich utilize the interaction between an electron stream and a travelingelectromagnetic wave to secure amplilication of the traveling wave, andwhich are now commonly designated as traveling wave tubes. Accordingly,the invention will be described with particular reference to suchtraveling wave tubes, although the principles of the invention areapplicable generally to devices which utilize the velocity modulation ofelectron streams and are thereby susceptible to noise space charge wavesof the kind described above.

In its usual form, a traveling wave tube is a vacuum tube in which anelectromagnetic wave is made to propagate along a slow wave circuit atthe same time that an electron stream is projected past the slow wavecircuit in coupling relation with the electromagnetic wave. To serve astheelectron source, there is provided an electron gun positioned beyondthe input end of the slow wave circuit. Such an electron gun customarilyincludes an electron emissive surface, or cathode, and an electrodesystem which includes beam forming and accelerating electrodes forfocussing the electron stream preliminary to its projection past thewave circuit. In the past, in order to achieve the maximum streamdensities along the path of ow past the Wave circuit, it has been commonto include an electron gun which has a cathode which produces initiallyan electron beam having a large cross section and an electrode systemwhich converges the ow into an electron beam of smaller cross sectionfor travel past the wave circuit. Alternatively, for simplification ofthe focussing problems, it has been a less frequent practice to utilizean electron gun which includes a cathode which provides initially a beamof the desired cross section for travel past the wave circuit and anelectrode system for maintaining plane electron llow.

It has now been found that, in traveling wave tubes where noise gure isan important consideration, it is advantageous to employ an electron gunwhich includes a cathode which provides initially a beam of small crosssection relative to that desired for projection past the wave circuitand an electrode system which diverges the ow into an electron beam ofthe desired cross sectional ditnensions and then collimates the electronbeam for plane ow past the wave circuit.

"ice

By utilizing divergent flow along most of the path between the cathodeand the input end of the wave circuit, it is possible to minimize theamplitude of the space charge waves which are set up by the thermaluctuations just oit the cathode and which propagate to the input end ofthe wave circuit and there act in the manner of input waves, therebyeiecting signal degradation. For example, calculations indicate that animprovement in noise level of approximately 6 db can be expected byproducing initially an electron beam of diameter substantially one-ninththat desired for the electron beam to be projected past the wavecircuit.

For example, for use in a traveling wave tube which employs a helix asthe slow wave circuit, the electron' source is designed to provideinitially an electron beam whose diameter is less than one ninth theinternal diameter of the helix and prior to projection past the helix,the flow is made to diverge until the electron beam attains a diametersubstantially equal to the internal diameter of the helix, and is thencollimatcd for travel past the helix.

The invention will be better understood from the following more detaileddescription taken in conjunction with the accompanying drawings inwhich:

Fig. l shows in schematic form a prototype electron gun for use indescribing the principles of the invention;

Fig. 2 is a longitudinal section of a helix type traveling wave tubewhich incorporates an electron gun suitable for providing divergent flowin accordance with the principles of the invention, andA which employsan electrostatic lens for collimating the electron stream;

Figs. 3 and 4 each shows fragments of traveling Wave tubes which includediterent forms of electron guns suitable for providing divergent ilow asis characteristic of the invention, and

Figs. 5 through 10 are graphical representations which will be useful ina description of the principles of the invention. l

Before describing in detail the specic embodiments of the inventionshown in Figs. 2, 3 and 4, it Will be advantageous to analyze theprinciples of the invention.

This will be done with reference to the prototype electron gun shownschematically in Fig. 1, which comprises essentially a cathode 11, abeam forming electrode 12, and an accelerating anode 13. In operation,the cathode is heated by suitable heating means (not shown) andelectrons are emitted from its surface. The electrons emanating from thecathode 1l are formed into an electron beam of desired configuration bythe action of the electrode system comprising the beam forming electrode12 and the accelerating anode 13. This action is deter-` mined by thegeometry and electrostatic eld characteristic of this electrode system.When the electron gun is embodied in a traveling wave tube, the electronbeam is projected beyond the anode 13 and past a suitable wave circuitunder the influence of accelerating elds as will be described more fullybelow. The existence in traveling wave tubes of noise space charge wavesset up by thermal fluctuations just off the surface of the cathode andwhich Vpropagate to the input of the wave circuit has beenexperimentally demonstrated. This work is found described in an articleby C. C. Cutler and C. F. Quate entitled Verication of transit timereduction of noise, Physical Review, vol. 80, pp S-878, December 1950.However, the nature of the noise space charge waves propagating in theaccelerating region from the cathode 11 to the accelerating anode 13 ofthe electron gun has not hitherto been adequately understood. Previousstudies have made use of equations governing space charge flow in a onedimensional plane diode. The actual beam, however, is generally twodimensional in cross section audequa1ly important, it has, in the past,generally converged in conical ow from the cathode 11 to the# anodeV 13of the electron gun.

The equations applicable to circularly cylindrical or conical flow in anaccelerating region are too complex for solution. However, the followinganalysis will solve the equations of electron motion in one dimensionalspherical coordinates so as to studythe effect of convergence; ordivergence of the beam. `By this approach, there can be obtained abetter understanding of the operationof tubes which employ twodimensional conical iiow. In this way, it isshown that divergent ow fromthe cathode 11 to the anode 13, as is contemplated for the practice 'ofthe invention, excites spacecharge waves in the drift region beyondanode 13 of considerably lessy amplitude than either plane or convergentflow.

In this analysis, there is studied the space charge waves .in sphericalcoordinates with'the D.C. velocity appropriate to space charge limitedflow. First, there are set up the equation of divergence for theelectric field, the continuity equation for the current density, and theforce equation, for thevelocity. These are, in turn,

where E=the electric tield acting on the electrons in lthe beam i=theconduction current density v=the velocity of electrons in the beam p=thecharge density in the beam f=the frequency under consideration e=thedielectric constant of the medium 17=the charge-mass ratio of anelectron Now, by postulating that Equations 1, 2, and 3 have wave-likesolutions, we look for such solutions by allowins,

E0f=D.`-C. electric field between the accelerating anode` and thecathodeV From. these relationships, it can be shownthat na 2 1 ,4mma2 7)where (it)2v isexpressed in series form by a=1nx-..3,(1nx)2+.075 1nxv)?+(8,) Equation-l 6 can be` written asV BMI 2 I/ 2 1 en 5; mit Talle 9)The solution of Equation 9 gives the nature of thespace charge waves:propagating in the cathode to anode regionf thezelectron guntor spacecharge limited flow.

U6 and the A.C. current i1 which is proportional to at!! .at

From Equation 7, it can be seen that un is proportional to (002/3.Accordingly, the A.C. velocity v1 is proportional to which is equal to t(aja/3;; In Fig. 7, v1 is shown plotted against x where x is the ratioof the radius of the electron beam at various points along, the lengthof the beam to its initial radius for and Ella' (002/a In Figs. 8 and 9,respectively, the A.C. current i1 is plotted against x for the functions@V 6x and.

for convergent andk divergent ilow, respectively.

It will be helpful to consider the noise space charge Waves set up bythe thermal uctuations at the cathode in a manner analogous to the planecase analyzed by J. R. Pierce, in his book Traveling Wave Tubes chapterX, Van Nostrand, New York (1950). In the plane case, it was shown thatin space charge limited ilow only the A.C. thermal velocity at thepotential minimum contributed to the space charge waves. Hence in thespherical case, since it is substantially equivalent to the plane casenear the cathode, we shall consider the boundaryy conditions at thecathode to include only A.C. thermal velocity. From Fig. 7, it can beseen that there'must be excluded the tbl solution since it becomesinnite at the cathode. Accordingly, only the tpg solution need beconsidered further. l In Fig. 10, there is plotted 1pz/am versus x,where as above, x is the ratio of the radius of the electron beam to theradius of the electron beam as it left the cathode. This gives the ratioof the A.C. velocity at the anode to the initialAfC. velocity at thecathode.V Forthe plane case, this ratio is unity (i.e. the velocity atthe vanode is equal in magnitude to the velocity at the cathode). Fig.yl0 further provides a direct comparison 0f the A.C. velocity at theanode for spherical geometry to thatfor plane geometry.

There can now be examined the space charge waves set up in the driftregion beyond the anode which propagate to the input en d of the wavecircuit. In Fig. 8, there is plotted all,

which is proportional to A.C. current density, for values of x greaterthan one, which is the divergng beam case. It is to be noted thattheA.-C. current for the 1pz solution approaches zero for values of Xnear 6. It can therefore be concluded that with a sufficiently divergngbeam the A.C. current at the anode can be neglected and only the A.C.velocity contributes to setting up the space charge waves in the driftregion beyond. Consequently, it is possible to consider thecharacteristic shown in Fig. as a measure of the ratio of the noisefigure of the spherical beam to that of the plane beam. Thus, forexample, we see that for a beam which diverges from a cathode so as tohave a diameter at the anode nine times that at the cathode the noisefigure is improved by a factor of 6 db, which represents a ratherconsiderable improvement.

Although the analysis set forth above has been specifically directedat'the case of a solid beam of circular cross section, the principlesestablished thereby are not necessarily limited to this specific kind offlow. In fact, the same improvement in noise figure can be attained forbeams of various other cross sectional forms, as, for example, beams ofrectangular cross section, or annular cross section.

Various forms of electron guns are known which are suitable forproviding divergent flow of the electron stream between cathode andanode for use in the practice of the invention. For example, UnitedStates Patent 2,268,196 issued to J. R. Pierce on December 30, 1941,discloses electron guns suitable for such divergent flow. Moreover, fora complete description of the design principles applicable to the designof suitable electron guns, reference is made to a book entitled Theoryand Design of Electron Beams by I. R. Pierce, published by Van NostrandCompany, Inc., New York (1949). However, it had not been appreciatedhitherto that in traveling wave tubes the advantage which can berealized in the form of improved noise figure by such divergent ow canoutweigh the disadvantages of lower stream densities and slightly moreexacting focussing requirements.

With reference now to specific embodiments in accordance with theinvention, in Fig. 2, there is shown, by way of example, a helix-typetraveling wave tube 20 which is characterized by an electron gun whichprovides divergent flow to the electron beam between the cathode and theaccelerating anode prior to its projection beyond the accelerating anodefor travel past the slow wave circuit. An evacuated envelope 21, whichfor example is of glass, has at the left hand end an enlarged endportion 22 wherein is housed the electron gun 23 and an elongatedportion 24 wherein is enclosed the helix wave circuit 25.

At the right hand end of the elongated portion, there is disposed atarget electrode 26 disposed in a collecting relationship for electronsprojected from the electron gun 23. The electron gun 23 and targetelectrode 26 define therebetween a path of flow for the electron beam,which is symmetric about the longitudinal axis of the envelope. Thehelix wave circuit 25 extends coaxial with this longitudinal tube axisin the path of flow.

The electron gun 23 comprises an electron source or cathode 31 and anelectrode system. As shown, the cathode is of the indirectly heated typeand comprises a metallic heater compartment 32 having a right hand endportion 33 thereof which is coated on the outside surface with asuitable thermionic material. and a heater filament 34 within thecompartment. To achieve a solid cir cularly cylindrical electron beamfor projection Within the helix wave circuit, the coated end portion iscircular. Alternatively, the thermionic material can he applied in anannular coating for providing a hollow circularly cylindrical electronbeam for travel past the helix Wave circuit. The electrode systemcomprises a beam forming electrode 35, a first accelerating anode36,'and a second accelerating anode 37, each supported in axialalignment with the cathode. The beam forming electrode 35 is a metallicelement which is apertured for passage of the electron beam therethroughand which extends transversely from the path of flow having aconfiguration which will provide suitable electrostatic fields. Aninsulating ring 3 9 keeps the cathode 31 and beam forming electrode 35spaced apart. By means of suitable voltage sources, a potentialdifference may be created between the cathode 31 and the beam formingelectrode 35 to effect a measure of control of the intensity of the beamcurrent. The first accelerating anode 36 is a circularly cylindricalmetallic jacket which is supported to fit around, although spaced apartfrom, the beam forming electrode 35. The anode 36 includes an end plate40 which extends transverse to the path of electron flow and has acircular aperture 41 of diameter substantially larger than the diameterof the thermionic cathode surface 33 for passage of the electron flowtherethrough. This first anode is main tained at a positive acceleratingpotential with respect to the cathode 31 and beam forming electrode 35.The beam forming electrode 35 and the first accelerating anode 36together form an electrostatic lens for the control of the electronbeam. The configuration of the left end surface of the end member 4l) ofthe accelerating anode 36 and right end surface of the beam formingelectrode 35, together with the accelerating potential being applied,determines the path of travel of electrons after emission from thethermionic cathode surface. In accordance with the invention, theelectrostatic lens formed by the beam forming electrode 35 and the firstaccelerating anode 36 is made diverging. In this embodiment, theappropriate surfaces are designed to make the electron beam divergealong this path of travel so that the circular electron beam which,initially as it leaves the circular thermionic surface 33, has adiameter substantially equal to that of the thermionic surface has, bythe time it passes through the circular aperture 41 in the end member 40of the accelerating anode 36, a diameter substantially larger, as forexample, nine times as large. The envelope of the electron ow has beenshown by the broken lines 42. The particular design principlesapplicable are well known to workers in the electron optics art, andreference can be had to the abovementioned Pierce book for a detaileddiscussion of these principles. The second accelerating anode 37similarlycomprises a metallic jacket which ts around, and is spacedapart from, the first accelerating anode 36. It is similarly providedwith an end plate 43 which extends transverse to the path of electronflow and has a circular aperture 44 for passage of the electron flowtherethrough in the manner of the rst accelerating anode. This anode ismaintained positive with respect to the first accelerating anode bymeans of suitable voltage sources. The anodes 36 and 37 together form anelectrostatic lens which colimates the divergent circular electron beaminto plane flow at substantially the increased diameter the beam hasattained by the time it passes throughvthe aperture 41 in the end plate40 of the first accelerating anode 36. To achieve this collimatingeffect, it is important that the configuration of the right end surfaceof the first accelerating anode 36 and the left end surface of thesecond accelerating anode 37 be properly designed. The design of such acollimating lens is well :known to workers in the electron optics art,and again, the principles can be found described in the above-mentionedPierce book. The sum elect of the electrode system is to transform thecircular beam of a diameter which is relatively small compared to theinternal diameter of the helix wave circuit, as it leaves the electronsource, into a circularly cylindrical beam of a diameter substantiallyequal to the internal diameter of the helix wave circuit for travelbeyond the apertured end plate of the second accelerating anode and pastthe helix wave circuit.

It is of course necessary to provide suitable supporting structure forthe various elements described, together with various lead-inconnections to establish the operating potentials necessary. However, inthe interest of simplicity, and since little would be gained by adescription of such details, these have been omitted.

To help keep the beam plane in its travel along the relatively longerpath of flow" beyond this plate, thereV maining portion of the path offlow is immersed in ai strong axial. magnetic field. For example, thestrengthv of this magnetic eld can be adjusted to provide Brillouin typeflow past the wave circuit. The principles of such ow can be founddescribed on pages 152 et seq. of the aforo-mentioned Pierce book; Bysuch flow, radial components acting on the stream are minimized. Thismagnetic field can be established, for example, by the solenoid 45disposed, as shown in Fig. 2, about the elongated portion 24 of, thetube envelope. To minimize the effect of this magnetic field on theoperation of the electron gun, it is advantageous to shield` theelectron gun from this field. To this end, there is provided anapertured transverse soft ironl plate 46 which fits around the tubeenvelope in alignment with the end plate 43 of the second acceleratinganode 37, which for such purposes similarly can be of soft iron. Thereres-ults a magnetic shield which keeps the electron gun. relativelymagnetic field free, while the remainder of the tube is immersed m thelongitudinal magnetic field.

The helix wave circuit 25, which is a plurality of wavelengths long atthe operating frequency, is positioned along a substantial portion ofthe path of iiow extending` beyond the accelerating anode 37. Thecircularly cylindrical electron beam preferably is confined to theiuterior of the helix, but owing closely past the turns of the helix.This helixA wave circuit can4 be. of the kind well known in thetraveling wave tube. Alternatively, it is contemplated that there may beemployed a multi-l pitch helix of the kind described in my copendingappli cation, Serial No. 220,416 filed April 11, 1951, now U.S. Patent2,908,844, issued October 13, 1959. Input waves are coupled to theupstream end of the helix by way of the input wave guide 47 and outputwaves are abstracted at the downstream end of the wave guide by way ofthe output wave guide 48. Both wave guides can be of conventionalrectangular cross section, and each is apertured for passage of the tubeenvelope therethrough. For improved coupling to the respective waveguides, it is advantageous to employ microwave transducers to effeetenergy interchange at the respective ends of the helix, which, forexample, can be of the kind described in United States Patent 2,575,383,which issued to L. M. Field on November 20, 1951. Such microwavetransducers customarily include a metallic coupling strip in conjunctionwith a helix portion of gradually increasing pitch. In operation theVhelix is maintained usually at the same potential ofthe secondaccelerating anode by a connection thereto which comprises a`non-magnetic conductive cylindrical sleeve 49, as shown in Fig. 2,.

The basic operation of the traveling wave tube is well understood, beingfully described in the afore-mentioned Field patent. Accordingly, sincethe basic operation` re,- mains unaffected by the use. of divergent flowas described, furtherV description thereof seems unnecessary. It is,however, true that although the same principles of operation areapplicable, there is effected a considerable improvement in noisefigure.

As should be evident to a Worker in the electron optics art, variousalternative arrangements can be employed for collimating the divergentelectron beam into a plane electron beam for ow past the wave circuit intubes constructed in accordance with the principles of the invention. Inparticular, Fig. 3 shows the electron gun portion of a traveling wavetube 120 which employs magnetic focussing for collimating thedivergentelectron flow. In other respects, this tube 120 is similar totube 2,6 shown in Pig. 2, and, accordingly, for simplicity, likereference numerals are used to employ corresponding elements, andrepetition of their various functions is avoided. However, theelectrostatic lens formed by the first and second accelerating anodes 35and 36 in the electron gun 23 of the tube 20, shown in Fig. 2, isreplaced by a magnetic lens.- For this purpose, for example, aA solenoid121 is provided, preferably disposed external to the tube envelope 21,along the initial portion. of the electron path extending beyond theapertured end plate 40 of the accelerating anode 36 of the electron gun.This solenoid 121 creates a longitudinal magnetic field along itsadjacent portion of the electron path, which combines with thelongitudinal magnetic field provided by the solenoid 45 which stillpreferably is used, as described above, to provide Brillouin type owpast the wave circuit. In the region where the fields of thetwosolenoids combine, the cumulative effect is strong enough to transformthe divergent electron flow into plane electron flow in accordance withprinciples found described in the above-mentioned Pierce book.Thereafter, the eldof the solenoid 45 keeps the electron flow plane forthe substantially longer remaining portion of the path of flow. As intube 20 of Fig. 2, an apertured transverse end plate of soft irondisposed external to the tube envelope is aligned with a soft iron endplate 40 of the accelerating anode 36 for shielding the electron gunfrom the magnetic fields beyond. The operation of the tube 120,embodying the kind of gun just described, is similar to that of tube 20in Fig. 2.

Fig. 4 shows an electron gun portion of a traveling wave tube whichemploys still another possible form of collimating arrangementconsistent with the principlesY of the invention. In this case,substantially the whole tube is immersed in a longitudinal magneticfield provided by an externally disposed solenoid 131. Again, theelectron gun comprises a cathode source 33 of an electron Stream ofrelatively small transverse dimensions and an electrode systemcomprising, as before, a beam forming e.ectrode and an acceleratinganode for forming the electrons emitted from the source into an electronbeam. In this case, by choice of beam accelerating potentials andmagnetic field strength in accordance with the principles set forth onpages 152 through 155 of the above-identified Pierce book which relatesto the Brillouin type case of a point source immersed in a longitudinaluniform magnetic field, the electrons can be formed into iiow which lsdivergent in the region between the cathode sour-ce 33 and theaccelerating anode 36, and plane for travel along that portion of theelectron path extending beyond the accelerating anode 36, as desired forthe practice of the invention. The wave circuit 25 again is positionedin coupling relationship with this plane electron beam.

It is to be understood that the various embodiments described` above aremerely illustrative of the principles of the invention. Still furtherarrangements can be devised by one skilled in the art without departingfrom the spirit and scope of the invention. For example, although theinvention has been described with particular reference to a travelingwave tube which employs a helix wave transmission circuit, theprinciples are applicable to traveling Wave tubes which employ variousother forms of wave transmissionv circuits. Moreover, it is possible toapply the principles of the invention to various other forms of velocitymodulation tubes; for example, to velocity modulation tubes of the kindknown in the microwave art as klystrons which employ standing waves formodulating the electron beam in its path of forward travel in accordancewith signal information.

It is to be noted that the expression plane when used in connection withan electron beam describes an electron beam which substantially neitherdiverges nor converges, and without reference to any specificcross-sectional conguration. Additionally the expression circularlycylindrical when used in connection with an elec.- tron beam describes aplane electron beam of substantially circular cross-section.

What is claimed is:

l. In a radio frequency device, an electron gun and a target electrodedefining therebtween a path of electron flow, the electron guncomprising an electron emissive surface, means for forming the electronflow emitted from saidsurface into a divergent electron beam, and meansfor collimating the divergent electron beam into a plane electron beamhaving an electron density less than onehalf the density of the electronflow at said emissive surface, means for maintaining the electron beamsubstantially plane throughout the remaining portion of the path ofelectron flow, and a wave transmission circuit positioned along asubstantial part of the remaining portion of the electron flow path incoupling relation with the plane electron beam for modulating the planeelectron beam in accordance with signal information.

2. In a radio frequency device, an electron gun and a target electrodedefining therebetween a path of electron flow, the electron guncomprising an electron emissive surface, electrode means for forming theelectron flow emitted from said surface into a divergent electron beam,and means for collimating the divergent electron beam into a planeelectron beam having an electron density substantially less than thedensity of the electron flow at said emissive surface, and magneticmeans for maintaining the electron beam substantially plane throughoutthe remaining portion of the path of electron flow.

3. In a radio frequency device, an electron gun and a target electrodedefining therebetween a path of electron flow, the electron guncomprising an electron emissive surface, electrode means for forming theelectrons emitted from said surface into a diverging electron beam, andmeans for collimating the divergent electron beam into a plane electronbeam whose axis is normal to the electron emissive surface, magneticmeans for maintaining the electron beam substantially plane throughoutthe remaining portion of a path of electron flow, and means along saidremaining portion of the path of electron ow for modulating the velocityof electrons in their path of forward travel in acordance with signalinformation.

4. In a radio frequency device, a source of electrons including anelectron emissive surface of predetermined area, electrode means forforming the electrons emitted from said surface into a divergentelectron beam, means for collimating said divergent electron beam into aplane electron beam having a cross-sectional area substantially greaterthan the area of said emissive surface, and means for velocitymodulating the plane electron beam in its path of forward travel inaccordance with signal information. Y

5. In a radio frequency device a source of an electron beam of a firstdiameter, means for forming said electron beam into a diverging electronbeam, means for collimating said diverging electron beam into acylindrical electron beam of a diameter at least several times the firstdiameter, and means for velocity modulating the electron beam in itspath of forward travel in accordance with signal modulation.

6. In a radio frequency device, a source of an electron beam of a firstdiameter, electrode means for forming said electron beam into adiverging electron beam, means for collimating said diverging electronbeam into a cylindrical electron beam of a diameter at least severaitimes the first diameter, and a wave transmission circuit positioned incoupling relation with the cylindrical electron beam for modulating thecylindrical electron beam in its path of forward travel in accordancewith signal information.

7. In a radio frequency device which utilizes the interaction between atraveling electromagnetic wave and an electron stream, an electron gunand a target electrode defining therebetween a path of electron flow,and a wave circuit positioned along said path for propagatingelectromagnetic waves in coupling relation with the electron flow, andcharacterized in that the electron gun comprises an electron emissivesurface of predetermined area, electrode means for forming the electronsemitted from said surface into a diverging electron beam, and means forcollimating the diverging electron beam into a plane electron beamhaving a cross-sectional area substantially greater than the area ofsaid emissive surface for projection past the wave circuit.

8. A radio frequency device according to claim 7 which includes meansfor immersing the wave circuit in a magnetic field extending parallel tothe path of electron ow for minimizing radial components of the planeelectron beam in its flow past the wave circuit.

9. In a radio frequency device which utilizes the interaction between atraveling electromagnetic wave and an electron stream, an electron gunand a target electrode defining therebetween a path of electron ow, anda wave circuit positioncd'along said path for propagatingelectromagnetic waves in coupling relation with the electron flow, andcharacterized in that the electron gun comprises an electron emissivesurface of predetermined area, a beam forming electrode, a first anodecooperatingwith said beam forming electrode for diverging the electronflow emitted from the electron emissive surface, and lens meanspositioned along the path of ow in the region between the emissivesurface and the wave circuit, for collimating the electron stream into aplane electron beam having a cross-sectional area substantially greaterthan the area of said emissive surface for travel past said wavecircuit.

10. A radio frequency device according to claim 9 in which the lensmeans includes a second anode.

11. A radio frequency device according to claim 9 in which the lensmeans includes magnetic means.

12. In a radio frequency device which utilizes the interaction betweenanelectron stream and a traveling electromagnetic wave, an electron gunand a target electrode defining therebetween a path of electron flow,and a helical wave circuit positioned along said path for propagatingelectromagnetic waves in coupling relation with the electron ow, andcharacterized in that the electron gun comprises an electron emissivesurface of pre determined area for providing a circular beam having adiameter substantially smaller than the internal diameter of the helix,beam forming means along the path for providing divergent flow for thebeam emitted from the emissive surface, and 'lens means along the pathfor collimating the beam to a cylindrical beam having a cross-sectionalarea substantially larger than the area of said emissive surface and ofa diameter substantially equal to the internal diameter of the helicalwave circuit for projection past the helical wave circuit.

13. A radio frequency device according to claim 12 in which the lensmeans includes electrostatic means.

14. A radio frequency device according to claim 12 in which the lensmeans includes magnetic means.

15. In a radio frequency device, a helix wave transmission circuit, asource of an electron beam including an electron emissive surface ofpredetermined area, said predetermined area being small relative to thecross sectional area of the cylinder bounded by the helix, means forforming said electron beam into a diverging electron beam, means forcollimating the diverging electron beam for forming a cylindricalelectron beam of cross sectional area substantially equal to that ofsaid cylinder, the helix being positioned along the path of fiow of thecylindrical electron beam, and means for immersing the path of low ofthe cylindrical electron beam past the helix in an axial magnetic eldfor minimizing radialcomponents of electron flow.

16. In a radio frequency device, means defining a longitudinal path ofelectron flow, said means including an electron emissive surface ofpredetermined area at one end of said path, an electrode system forforming the electrons emitted from said surface into a divergingelectron beam for travel along a'relatively short portion of said path,a lens system for collimating said diverging electron beam into a planeelectron beam having a crosssectional area substantially greater thanthe area of said emissive surface, magnetic means for maintaining theelectron "beam substantially plane along a relatively long;

portion of' said path and means for modulating the plane electron beamin accordance with signal information.

17. A radio frequency device according to claim 16 in which the meanslfor modulating the plane electron beam is a wave transmission circuitpositioned along tlie relatively long portion of the path forpropagating signal information. l

18. In a radio frequency device, means defining a longitudinal path ofelectron flow, a source'of electrons at one end of the path, anelectrode system for forming the electrons into a diverging electronstream for travel along a relatively short initial portion of the path,a lens system for collimating said diverging electron beam into a solidVcircularly cylindrical electron beam for travel along a relativelylonger portion of the path, and a helix wave circuit positioned alongsaid relatively longer portion of -the path in coupling relation withthe solid circularly cylindrical electron beam, the internal diameter ofthchelix being substantially equal to the diameter of the solidcircularly cylindrical electron beam and at least twice the diameter ofsaid circular electron beam at the source end of the path'.

19. In a radio frequency device which utilizes the interaction between atraveling wave and an electron beam, means including a cathode, a beamforming electrode, and an accelerating electrode spaced apart in thedirec tion of electron flow from said cathode, for forming a divergentelectron beam whose electron density decreases with increasing distancefrom said cathode, the crosssectional area of the beam at theaccelerating electrode being at least double the beam cross-sectionalarea at the cathode anda wave guiding structure positioned along thepath of ow beyond said accelerating electrode for propagating thetraveling wave.

2t?. In a radio frequency device which utilizes the interaction betweenan electromagnetic Wave and an elec tron beam, means including in spacedsuccession, a cathode, a beam forming electrode, and an acceleratingelectrode, for forming a divergent electron beam whose electron densitydecreases progressively with increasing distance from said cathode, thecross-sectional area of said beam at the accelerating electrode being atleast nine times its cross-sectional area at the cathode, and meanspositioned along the path of iiow beyond said accelerating electrode forvelocity modulating the electron beam.

1I. An eletren gun for producing-a solid; cylindrical Brillouin flow ofelectrons having a substantially constant predetermined nolcross-sectional area along a predetermined path, said gun comprising asource of electrons disposed on said path for producing an electronstream having a predetermined initial cross-sectional area; means forproducing un axial magnetic ,field along said path extending throughsaid source; and means including an electron lens for directingelectrons away from said source at a velocity to allow the space chargeof said stream to expand the volume occupied by said stream outwards tou volume having a predetermined final circular crossscctionol areaseveral times greater than said predetermined initial cross-sectionalarea of said electron stream whereby the outward expansion of saidelectrons in saidr uxiul magnetic jeld causes them to rotate about thelongitudinul axis of said path. A 7

22. An electron gun for producing a solid, cylindrical, Brillouin flowvof electrons having a substantially constant predetermined ,nalcross-sectional area along a predetcrmined path, said gun comprising asource of electrons disposed on said path for producing an electronstream, said stream having a predetermined initial cross-sectional area:means for producing an axial magnetic field along said path extendingthrough said source; means for' accelerating said electrons uw@ fromsaid source along a portion of said path ai a velocity to allow thespace charge of said stream to expand said stream outwards in said axialmagnetic field whereby said stream electrons are caused to rotaie aboutthe longitudinal axis of said path; and means for producing an electronlens' disposed subsequent to said portion of said path at a point wherethe radial forces acting upon said stream electrons arc in equilibriumto reduce the radial velocity components of said stream electrons tozero zt-said point', thev cr0ss-sec tional area of said stream beingequal at said point to said predetermined jnal crosssectional area.

References Cited in the ille of this patent or the origlnal patentUNITED STATES PATENTS,

2,575,383 Field a Nov. 30, 1951 2,578,434 Lindenbladv Dec. 1l, 195i2,608,668 y Hines Aug. 26, 1952 2,632,130 Hull Mar. 17, 1953 2,817,035Birdsall Dec. 17, 1.957

